Small hydro is not just about dedicated plants in remote areas. There are slots in the UK's extensive water supply network where a turbine would be a positive aid to the system, says Janet Wood
IS WATER for power different from water for the tap? Not really. The water source that ultimately gets to our taps is in high upland areas which have high rainfall and a wide catchment area. The water is piped from its source to the water treatment works near the users and that means that at the treatment works it is under high pressure – the head may be hundreds of meters. But this is not the reason water is supplied under pressure to the tap: after being treated, water is stored in huge tanks and that provides the water pressure at the point of delivery. In fact before it enters the treatment system the water may be under too much pressure and it has to pass through special ‘pressure reducing’ valves.
But head and flow are the components for hydro and the effect of a hydro power turbine is to remove energy from the water and turn it into electricity. Why not replace the pressure-reducing valve with a hydro turbine? This idea is not new. For many years engineers have looked at the energy wasted in pressure reducing valves and considered how best to extract it. A UK-wide assessment of hydro potential carried out by the University of Salford in 1989 (ETSU SSH-4063, Parts 1-3) assessed the entire potential of the UK’s water supply system at 18.8MW. This is a fairly small amount, especially as it would be dispersed throughout the country in installations usually sized at much less than a MW, but in aggregate, it provides enough power for up to 20,000 houses – the equivalent of small town.
According to the British Hydropower Association (BHA) the Salford figures are out of date and the potential is much larger. It says that schemes already in operation in the water system provide over 25MW – more than the total potential found by the Salford report. In some regions the disparity is startling: the Salford report found 0.5MW of potential hydro power in the southwest, excluding hydro sites, but there is now 4.1MW identified or installed, and the BHA pegs the theoretical potential in the region at 12.5MW. Similarly in western Scotland, the Salford report found 500kW of non-river hydro, but the potential is now thought to be 3.5MW. Overall, the BHA says the potential nationwide is likely to be around 100MW – over five times the amount identified in 1989 and more than any one of the offshore wind farms currently winning government support.
Energy from the water system may seem like easy pickings. But describing projects under way in the South West, South West Water’s Steve Cryer – who is also chairman of the BHA – explains that there are still conflicts to be rationalised. He says that the drinking water regulations are strict, and for good reason, and water companies see their business as running water supply, and are very wary about exploiting the potential for power.
Cryer explains some of the technical issues that arise when considering whether to introduce a turbine into what, in some areas, is a very old water supply system, and one that always has very strict operating requirements.
All electricial components in the water supply system have to fulfil a special set of regulations (the Water Industry Mechanical & Electrical Specification 3.05, issued in December 2000 and with its own dedicated website at www.pumpcentre.com). ‘The industry is reticent about mechanical components,’ says Cryer. ‘We have to ensure that there is no possibility of contamination by paint, oil or grease.’
The turbine is being added to an existing system so there will usually be very tight physical constraints on the site and Cryer points out that in many cases the existing pipework is very old. ‘The treatment works have to run constantly so there can be no shutdown when the turbine is installed or maintained,’ he said. Once in operation, the water flow must be tightly controlled – the rate must not change by more than 1.5% per minute so that water quality is maintained. This usually requires a sophisticated control system for the turbine and special control arrangements that allow water to be switched to bypass the turbine when necessary. A bypass system will be used if the turbine needs to be shut down for maintenance, or during normal operation to ensure that the flow is constant. Switching between the turbine and bypass route must be ‘bumpless’ Cryer explains – ie it must not create surges in the system. There is also a potential for surges within the penstock as flow from the source is not always constant.
In the last resort, engineers should realise, says Cryer: ‘The turbine may be sacrificial – we may have to allow it to be damaged to ensure the water supply is not compromised.’.
Waste water turbines
While South West Water is installing turbines in the intake to its system, at least one other water utility has been looking further downstream. Pressure reducing valves are also used in the waste water system and Southern Water, for one, has already investigated the feasibility of installing hydro turbines.
The company had considered installing them at its Portsmouth and Havant Wastewater Treatment Works and Recycling Centre at Budds Farm in the UK, which has recently undergone refurbishment and extension. But planning for that project was completed before the advent of renewables obligation certificates (ROCs) and at that stage it was considered uneconomic to proceed. A second plant was also considered uneconomic. But the projects were not shelved entirely, according to Southern Water’s energy manager, Martin South: ‘Powering our headquarters with our own green electricity is part of our commitment to a sustainable future. We recognise the importance of green electricity to the environment.’
With the introduction of ROCs, the company now plans to look again at the business case for its small hydro scheme.